Equipment for controlling electrical power to or from other electrical equipment, such as the load tap-changer, no load tap changer or other switches in a power transformer, often includes mechanisms that direct the flow of high current between relatively fixed and movable electrical terminals or points in the equipment. Because electrical equipment takes on many forms, various solutions to the problem of transfering high current between two points, one of which is movable, have been offered by those skilled in the art. In mechanisms where a movable point or terminal rotates less than a complete revolution about an axis that is parallel to or coincident with an axis passing through a fixed point or terminal, an important design consideration is the torsional force developed in the electrical connection joining together the two terminals.
In addition to the torsional force consideration, there is the consideration of the electrical resistance of the connection between the two terminals or points. When high currents are involved, the electrical resistance consideration becomes very important. Thus, the electrical resistance of the connection between the two points should be as low as possible for a minimum power dissipation. It should be appreciated, however, that as torsional force increases better electrical contact is obtained. Thus, torsional force and electrical resistance are opposing considerations. What is needed is to optimize these two variables for a particular mechanism.
There is still another consideration. Compactness is always a desirable characteristic in electrical equipment. Not only is there material savings but, as the equipment becomes smaller, it is more easily located and capable of being used in a greater variety of situations. However, it often becomes difficult to provide a high current capability within a small volume, especially when high voltages are involved.
Various solutions have been proposed to the problem of providing a high current capability and a minimum torque load in a small volume. Most of these designs have employed "spring-loaded" contacts. An elementary design is described in U.S. Pat. No. 1,395,886; in that patent a rotatable arm carries two spring-loaded contacts which bridge the path between two sets of terminals. Similar designs are illustrated in U.S. Pat. Nos. 1,649,107 and 2,760,017. A circular cluster of contact elements, which are used to shunt two terminals or points which move relative to each other, is illustrated in U.S. Pat. Nos. 3,636,290 and 4,315,122. Spring-loaded contact assemblies are also shown in U.S. Pat. No. 3,959,616; a modern design is shown in U.S. Pat. No. 3,739,120. The relatively large size of such mechanisms when used in load tap changing transformers, particularly the three phase variety, is shown in U.S. Pat. No. 2,513,953. Even in those devices which incorporate a sliding contact arrangement, a continuous or unbroken connection is preferred to transfer current to at least one of the contact elements. Typically, braided strands of electrically conductive material (such as copper, aluminum or the like) are used to make this connection. This connection has a very low electrical resistance and, more importantly, essentially avoids the torque loading problem previously described. One difficulty with this design is that the cross-section often must be very large in order to handle the large currents which must be provided for. Some examples are provided in U.S. Pat. Nos. 3,143,621; 3,729,608; and 4,205,209. One relatively modern design is illustrated in U.S. Pat. No. 4,280,030. In this patent a flexible connection is provided by plurality of laminations which are formed in the shape of the letter "S". High current capability is provided by the laminations and torque loading is minimized. However, only a relatively limited degree of rotation or movement is possible. More importantly, the mechanism is still relatively large. Thus, the problem of providing a compact, efficient, low torque, low-resistant high current carrying capability between a set of contacts which move relative to each other in a high voltage environment remains to be solved.